An image sensor is a device for converting an optical image to an electric signal. Image sensors may be categorized generally as complementary metal-oxide-silicon image sensors (CMOS) and charge coupled device CCS image sensors.
Comparatively, CCD image sensors may exhibit enhanced photosensitivity and lower noise than CMOS image sensors but has difficulty achieving high integration density and low power consumption. On the contrary, CMOS image sensors has simple manufacturing processes and may be more suitable for achieving high integration density and low power consumption.
Aspects of semiconductor device fabricating technology have focused on developing CMOS image sensors due to improved fabricating technology and characteristics of CMOS image sensors. Each pixel of a CMOS image sensor may include a plurality of photodiodes for receiving light and a plurality of transistors for controlling inputted video signals.
CMOS image sensors may be categorized in accordance with the number of transistors, such as a 3T-type, a 4T-type, etc. A 3T-type CMOS image sensor may include a photodiode and three transistors while a 4T-type image sensor may include a photodiode and four transistors.
As illustrated in example FIG. 1, a 4T-type CMOS image sensor may include photodiode region PD formed on and/or over a widest portion of active region 1 and transfer transistor Tx, reset transistor Rx and drive transistor Dx overlapped with a portion of active region 1 except photodiode region PD.
As illustrated in example FIG. 2, CMOS image sensor may include epitaxial layer 4 formed on and/or over semiconductor substrate 2 divided into active region 1 and device isolation region. Semiconductor substrate 2 may be a p++ type substrate and epitaxial layer 4 may be a p− type.
The CMOS image sensor may further include device isolation layer formed in the device isolation region of semiconductor substrate 2, gate electrode 12, gate oxide layer 16, gate spacers 18, n− diffusion region 22, LDD region 24, source/drain regions 26, 28, insulating interlayer 32, first contact hole 33, second contact hole 35, first contact plug 34 formed in the first contact hole 33, and a second contact plug formed in second contact hole 35, and metal line 36.
Gate electrode 12 for transfer transistor Tx and reset transistor Rx may be formed on and/or over epitaxial layer 4 including gate insulating layer 14. Gate oxide layer 16 and gate spacer 18 may be formed on both sidewalls of gate electrode 12 and gate insulating layer 14. n− diffusion region 22 may be formed in epitaxial layer 4 of photodiode region PD. LDD region 24 may be formed in active region 1 between transistors Tx, Rx, and Dx. Source/drain region 26, 28 may be formed on and/or over LDD region 26 next to both sides of gate spacer 18. Insulating interlayer 32 may be formed on and/or over epitaxial layer 4 to cover gate electrode 12 including gate spacer 18.
First contact hole 33 may be formed in insulating interlayer 32 to expose source region 26. The second contact hole may be formed on and/or over a gate electrode of drive transistor Dx. First contact plug 34 and the second contact plug may be formed in first contact hole 33 and second contact hole 35 of insulating interlayer 32, respectively. Metal line 36 may be formed on and/or over one of first contact plug 34 and the second plug to electrically connect source region 26 and the gate electrode of drive transistor Dx.
In operation, photodiode PD may sense incident light and then generates charges according to light intensity variation. Transfer transistor Tx may transfer the charges generated by photodiode PD to floating diffusion region FD. Before the transfer, floating diffusion region FD transports electrons from photodiode PD to reset transistor Rx to turn on. Thus, floating diffusion region FD may be set having a low charge state at a predetermined level. Reset transistor Rx may discharge charges stored in floating diffusion region FD for signal detection and drive transistor Dx may serve as a source follower for converting the charges to a voltage signal.
In the above-configured CMOS image sensor, if the charges generated from photodiode region PD are transferred to floating diffusion region FD, reset transistor Rx is turned on. Thus, the charges are sent to drive transistor Dx and then converted to a voltage signal. Floating diffusion region FD may function as a capacitor. Moreover, a junction cap may form between source/drain regions 26, 28 heavily implanted with n+ impurity ions and the p− type epitaxial layer 4. The junction cap mainly controls capacitance with doping concentration. Yet, may be a problem as causing low capacitance. Specifically, floating diffusion region FD may receive electrons from photodiode region PD and then convert the electrons to a voltage. If low capacitance occurs, noise sensitively may become a problem.